1. Field of the Invention
The present invention relates generally to sensing the level of a first form of energy and converting it to a corresponding value in a second form of energy and more particularly to doing so with an array of transducers.
2. Description of the Related Art
When measuring a given phenomenon, for example measuring pressure with a transducer which converts a change in pressure to a related change in electrical resistance, it is desirable that the conversion be smooth and repeatable. In many applications, such as pressure operated controls or process sensors, a linear transducer response is also desirable.
Prior art transducers have a number of inherent disadvantages. Several methods have been developed in the prior art to compensate for the disadvantages. For example, electrical circuits are used to filter noise, to average signals which waiver over time and to linearize the response of the transducer. Such circuits add expense, as well as limitations of their own, to the combined response of the transducer and associated compensating circuitry.
U.S. Pat. No. 4,314,227 to Eventoff discloses an electronic pressure sensitive transducer which produces a decreasing resistance responsive to increasing pressure applied to the transducer. In Eventoff, two sets of relatively conductive contact fingers are interleaved with one another. The interleaved fingers are pressed against a less conductive facing layer. So doing forms a plurality of parallel connections between the two sets of interleaved fingers through the resistive surface. When the pressure applied to the transducer is relatively low, the number of contacts formed between the fingers through the facing layer is also low. In this operating range, the transducer exhibits an unrepeatable, nonlinear response. In this range the transducer is also relatively noisy. A.sup.s pressure applied to the transducer increases, the number of connections between the fingers also increases. In this range the response is more repeatable although still nonlinear until fairly high pressures are applied.
Because each of the connections between the interleaved fingers through the facing layer are electrically in parallel with one another, a single low resistance connection can dominate the response of the transducer unless the number of connections is very large. As noted above, when a relatively low pressure is applied to the transducer, the number of connections is not large. In the presence of one or more low resistance connections, transducer response tends to be noisy and very nonlinear. Such low resistance connections may arise in a number of ways, such as variations in the surface finish of the interleaved fingers, variations in the resistance of the facing layer or as a result of physical damage to the transducer. Increasing the number of interleaved fingers increases the number of potential connections and thus reduces the problem. In practice, however, there is a limit to the number and separation of the fingers that can be reliably produced without occasional shorting between the fingers. Such shorting renders the transducer useless.
U.S. Pat. No. 4,897,629 to Lecklider discloses a variable control device comprising a conductive elastomeric material urgeable against an adjacent resistive layer. As the elastomeric material is pressed onto the resistive layer, it shunts a progressively larger area of the resistive layer thereby lowering the resistance between two electrodes connected to opposite edges of the resistive layer. Such a transducer exhibits a range of responses depending upon the resistances of the elastomer and the resistive layer as well as the shapes of the elastomer and the surface of the resistive layer where they touch one another. Relatively low noise, linear responses may be obtained when the resistances of the elastomer and resistive layer as well as the shapes thereof are carefully controlled.
The Lecklider transducer, as well as other transducers utilizing elastomeric material, suffer from several disadvantages. Conductive elastomers exhibit creep effects over time which inject hysteresis into the transducer response. Temperature variations change the elasticity of the material and therefore the response of the transducer. In addition, the elastomeric material must be formed into selected shapes. Regardless of the shape, contacts between the elastomeric material at the leading and trailing edges thereof dominate transducer response. Careful control of the shape contacting the resistive layer is therefore required for good response. In order to deform the elastomeric material, movement is required. In many applications, however, no movement or very little movement of the transducer or the transducer actuator is desired.
U.S. Pat. No. 4,479,392 to Froeb et al. discloses a force transducer which produces a varying resistance responsive to varying force applied thereto. The Froeb et al. transducer includes a planar low resistant shunt opposite a planar resistive surface. Both the shunt and resistive area are formed from inks silkscreened onto opposed planar insulating substrates. One of the substrates is a film against which an actuator is movable in order to urge progressively increasing surface areas of the resistive layers together responsive to increasing force. A conductive elastomer is not required but careful control of the shape of the actuator is necessary. The opposing resistive layers must be spaced apart to insure that the only area of contact is that resulting from force applied to the actuator.
A linear relationship between force applied to the actuator and the surface area of the contact created is desirable, but difficult to achieve in practice. A further disadvantage is that only the contact points along the leading and trailing edges of the contact area determine transducer response. Care must therefore be taken to control the progression of leading and trailing edges responsive to applied force. Appreciable movement of the actuator is required which, as noted above, is not desirable in many sensing applications. Because the actuator is preferably formed from elastomeric material, the relationship between actuator force applied to the actuator and the shunted surface area may change with time and temperature.
Another problem in prior art transducers is a lack of control of manufacturing parameters which can affect sensitivity and response characteristics of the transducer. Such parameters may include resistance between the contacts made by the opposing layers, shape of the contact or the resistance of an elastomeric material. It may be difficult to control these parameters closely, especially in high volume manufacturing. It would be desirable to have easily controlled manufacturing parameters which can be easily varied to define a desired transducer response.